Geochemical characterization evidence for the climate variability of the Mid-Pliocene warm period in the Nihewan Basin, North China

The mid-Pliocene warm period (mPWP) is regarded as an important geological analogue for predicting future climate characteristics and trends, due to its similar atmospheric CO2 concentration to that of the modern period. Within a chronologic framework provided by magnetostratigraphy, we analyzed the carbonate delta O-18 and geochemical element contents of 243 sediment samples from the NHA drill core in the Nihewan Basin, and combined them with grain size and magnetic susceptibility records. Our results revealed the paleoclimatic characteristics in the Nihewan Basin during 3.66-2.89 Ma for the first time. The interval of 3.58-3.31 Ma had the warmest and most stable climate during this interval, with low-amplitude fluctuations in all proxies, and with the following general characteristics: delta O-18 <-8 parts per thousand, CIA > 40%, Rb/Sr > 0.3, Sigma REE > 180 mu g/g, and TiO2 > 0.7%). The climate remained warm overall during 3.31-3.10 Ma, but there were three pronounced cold events (at similar to 3.30 Ma, similar to 3.23 Ma, and similar to 3.14 Ma) with the following general characteristics: delta O-18 > -7 parts per thousand, CIA < 35%, Rb/Sr < 0.2, Sigma REE < 160 mu g/g, TiO2 < 0.6%). These events are correlative with increases in global ice volume, corresponding respectively to Marine Isotope Stage (MIS) M2, KM6, and KM2. The climate cooled substantially after 3.10 Ma, and most of the proxy index values were close to those during the three previous cold events. We ascribe this to increases in global ice volume and the strengthening of the East Asian winter monsoon.